This invention relates to radiation emission devices in general, and in particular to radiation emission devices of the type which are frequently referred to as lasers. Lasers are generally characterized by an elongated envelope containing a material which can be raised from an initial energy state to a so-called excited energy state. The particular means used to excite the material in the envelope may vary. Thus, depending on the type of laser used, optical, electrical or chemical excitation means may be employed.
After excitation, radiation may be emitted spontaneously as the excited material returns to a more stable energy level, and/or through stimulated emission. In either case, the wavelength of the radiation so emitted is proportional to the energy difference between the energy levels of the transition involved. This, in turn, depends upon the inherent characteristics of the material itself.
The radiation, which propagates at a constant wavelength, generally leaves the envelope via radiation transmission means disposed at both ends thereof. The radiation transmission means are typically tranlucent windows which are often, but not necessarily, inclined at an angle which optimizes a particular polarization of light. This inclination is usually referred to as Brewster's angle, and the windows so inclined are often characterized as Brewster's windows.
Lasers of the type described typically include reflection means such as concave mirrors located at predetermined distance beyond each translucent window. The mirrors are aligned such that the radiation emitted from a translucent window is reflected back into the envelope to stimulate the emission of a substantially increased amount of radiation which then passes through the opposite window. This increased radiation is likewise reflected back into the envelope by the other mirror, thereby increasing the emitted radiation even more. As the radiation is continuously reflected back and forth through the envelope, greater and greater amounts of radiation are produced. It is in this manner that the energy of stimulated emission of radiation is "amplified" by the laser device. Of course, in order to enable the amplified radiation to escape therefrom, at least one of the mirrors are generally made only partially reflective.
Many different materials may be used to effect radiation emission, including certain members of the class of materials known as metals. Because the metals used in this type of laser must generally be transformed from a normally solid or liquid state, to a gaseous state in order to effect excitation, such lasers are frequently referred to as metal vapor lasers. It is thus clear that in metal vapor lasers, excitation means must be provided which first vaporize the metal and then raise the vaporized metal from an initial energy state to an excited energy state.
In general, gas lasers may be categorized into two main configuration types; a positive column type laser as described, for example, in U.S. Pat. No. 4,187,474 and; hollow cathode lasers as described, for example, in U.S. Pat. Nos. 4,021,845 and 4,052,680. The hollow-cathode laser tubes generally provide a higher gain for a smaller tube size for the same output than a positive column type laser device.
Although the negative glow discharge hollow cathode lasers disclosed in the latter mentioned patents provides very satisfactory performance, improvement on the overall laser efficiency by reducing the required input power for threshold laser action would be desirable.